Cleanup: Sequencer: Replace seqbasep variable access with function

With the aim of removing `seqbasep` to remove the complicated logic for
repairing pointers within the `Strip` struct when loading files and undo
steps, this commit just moves access of the variable behind a function.
In the future the function will retrieve the list from a Strip pointer,
for now it just returns the existing pointer.

Overall motivation is that blend file pointer manipulation is incompatible
with the changes required for #127706.

Pull Request: https://projects.blender.org/blender/blender/pulls/144624

tests/python/sequencer_load_meta_stack.py

@@ -26,7 +26,7 @@ class SequencerLoadMetastaskTest(unittest.TestCase):
         self.assertEqual(len(meta_stack[0].sequences), 1)
         self.assertEqual(meta_stack[0].sequences[0].name, "Color")
 
-        # accesses ed->seqbasep through screen_ctx_selected_editable_sequences
+        # accesses ed->current_strips() through screen_ctx_selected_editable_sequences
         bpy.context.copy()